(CNN) -- On three occasions since January, Sen. Barack Obama's passport file was looked at by three different contract workers, said State Department spokesman Sean McCormack.

Sen. Barack Obama's passport file was breached three times since January, the State Department said.

The contractors accessed information in the file in an unauthorized way, he said.

Two contractors were fired and one was disciplined by the contractor's company, McCormack said.

He said the contractors are not linked.

The State Department hires contractors to design, build and maintain their systems and help employees with searches. McCormack said two of the contractors in the Obama case were "low-level" personnel and the other was in a mid-level position with no management role.

The breach seems like "imprudent curiosity" among the contract workers, said McCormack, adding that senior management at the State Department was not aware of the incidents until Thursday afternoon. Breaches occurred January 9, February 21 and March 14.

Obama's campaign is asking for a complete investigation to find out who looked at Obama's passport file and why.

"This is an outrageous breach of security and privacy, even from an administration that has shown little regard for either over the last eight years," said Obama campaign spokesman Bill Burton in a statement.

"Our government's duty is to protect the private information of the American people, not use it for political purposes."

Doug Hattaway, a spokesman for Sen. Hillary Clinton, Obama's rival for the Democratic presidential nomination, said, "If it's true, it's reprehensible, and the Bush administration has a responsibility to get to the bottom of it."

The White House declined comment Thursday evening, just hours after the State Department upper management learned of the breach.

State Department officials say Secretary of State Condoleeza Rice was told Thursday what happened and that she told her staff she wanted a full investigation.

"That obviously is something we are investigating," said Under Secretary of State Pat Kennedy. "I have no reason to believe they did, but I certainly am not going to be dismissive of what is a serious and valid question."

Kennedy said he will brief Obama's senior staff on Friday.

The news was reminiscent of a breach of Bill Clinton's passport information during the 1992 presidential campaign. The FBI launched an investigation after the State Department reported that someone had ripped out pages from his passport file from the late 1960s and '70s.

The department concluded that a search of Clinton's passport records was an attempt to influence the presidential election, reportedly by trying to show that Clinton tried to seek citizenship in another country to avoid the draft. Clinton was running against President George H.W. Bush.

Then-State Department Inspector General Sherman Funk found no evidence the White House ordered department staffers to dig for political dirt in Clinton's passport files. However, Funk said the White House probably knew it was happening.

"I am deeply troubled that State Department contract employees sought access to Sen. Barack Obama's passport files. Firing or disciplining those responsible is an important first step. But we need to understand why these employees had access to this information in the first place, why they sought the information, and why it took over two months for this matter to come to light. I urge the Secretary of State to promptly refer this matter to the State Department Inspector General for investigation."E-mail to a friend

CNN's State Department correspondent Zain Verjee contributed to this report.